This invention relates to a communication system and to a handover communication method thereof. More particularly, the invention relates to a communication system, which is equipped with a mobile station, base station and base station controller, for sending and receiving user data via two or more transmission paths between the base station controller and mobile station at the time of handover, and to a handover communication method.
A wireless communication system that employs the W-CDMA scheme has been standardized by the 3GPP (3rd Generation Partnership Project), and actual service has begun in Japan as well. FIG. 34 is a schematic view of the configuration of a wireless communication system. A radio access system (RAN: Radio Access Network) in 3GPP comprises an RNC (Radio Network Controller) 1, base stations (Node B's) 3a, 3b, . . . and UE's (User Equipment: mobile stations) 5, 6, . . . . A base station controller 1 is connected to a CN (Core Network) 7 by a Iu interface, and the base stations 3a, 3b are connected to the base station controller 1 by Iub interfaces.
In such a mobile communication system compliant with 3GPP specifications, transmission power control is carried out in such a manner that a prescribed error rate is obtained at the base stations 3a, 3b and mobile stations 5, 6 and in such a manner that transmission power will not become excessive. FIG. 35 is a diagram for describing such transmission power control (inner-loop control) and illustrates a case where the transmission power of a base station is controlled.
A spread-spectrum modulator 3a1 of the base station 3a spread-spectrum modulates transmit data using a spreading code conforming to a specified channel. The spread-spectrum modulated signal is subjected to processing such as orthogonal modulation and frequency conversion and the resultant signal is input to a power amplifier 3a2, which amplifies this signal and transmits the amplified signal toward the mobile station 5 from an antenna. A despreading unit 5a in the receiver of the mobile station applies despread processing to the receive signal and a demodulator 5b demodulates the receive data. A SIR measurement unit 5c measures the power ratio between the receive signal and an interference signal and a comparator 5d compares target SIR and measured SIR. If the measured SIR is greater than the target SIR, the comparator 5d creates a command (a down command) that lowers the transmission power by TPC (Transmission Power Control) bits. If the measured SIR is less than the target SIR, the comparator 5d creates a command (an up command) that raises the transmission power by the TPC bits. The target SIR is a SIR value necessary to obtain, e.g., 10−3 (error occurrence at a rate of once every 1000 times). This value is input to the comparator 5d from a target-SIR setting unit 5e. A spread-spectrum modulator 5f spread-spectrum modulates the transmit data and TPC bits. After spread-spectrum modulation, the mobile station 5 subjects the signal to processing such as a DA conversion, orthogonal modulation, frequency conversion and power amplification and transmits the resultant signal toward the base station 3a from an antenna. A despreading unit 3a3 on the side of the base station applies despread processing to the signal received from the mobile station 5, and a demodulator 3a4 demodulates the receive data and TPC bits and controls the transmission power of the power amplifier 3a2 in accordance with a command specified by the TPC bits.
FIGS. 36 and 37 are uplink and downlink frame structures, respectively, standardized by 3GPP. “Down” indicates the direction in which a base station transmits data to a mobile station. Conversely, “up” indicates the direction in which a mobile station transmits data to a base station.
As illustrated in FIG. 36, the uplink frame has a dedicated data channel (Dedicated Physical Data Channel: DPDCH) on which only transmit data is transmitted, and a dedicated control channel (Dedicated Physical Control Channel: DPCCH) on which a pilot and control data such as TPC bit information are multiplexed and transmitted. After each of these is spread by an orthogonal code, they are mapped onto real and imaginary axes and multiplexed. One frame on the uplink has a duration of 10 msec and is composed of 15 slots (slot #0 to slot #14). Each slot of the dedicated control channel DPCCH consists of ten bits, the symbol rate is 15 ksps, and the slot transmits a pilot PILOT, transmission power control data TPC, a transport format combination indicator TFCI and feedback information FBI. It should be noted that DPCCH and DPDCH are both referred to as a DPCH (Dedicated Physical Channel).
As illustrated in FIG. 37, the downlink frame is such that one frame is equal to 10 msec and is composed of 15 slots #0 to slot #14. A dedicated physical data channel DPDCH that transmits a first data portion Data1 and a second data portion Data2 and a dedicated physical control channel DPCCH that transmits PILOT, TPC and TFCI are time-division multiplexed slot by slot. The dedicated physical data channel DPDCH has {circle around (1)} a dedicated traffic channel DTCH and {circle around (2)} a dedicated control channel DCCH. The dedicated traffic channel DTCH is a channel that transmits dedicated traffic information between the mobile station and network, and the dedicated control channel DCCH is a channel used in the transmission of dedicated control information between the mobile station and network.
The foregoing relates to a case where one mobile station is communicating with one base station. At the time of handover due to travel, however, the mobile station 5 communicates with two or more base stations 3a, 3b simultaneously, as illustrated in FIG. 38. At the time of such handover, the base station controller 1 selects the uplink data of the best quality from the uplink data received from the plurality of base stations 3a, 3b. Selecting the data having the best quality is referred to as “selective combining”, and such control at the time of handover is referred to as “diversity handover (DHO)”.
DHO is not limited to the uplink and is performed similarly also on the downlink. As shown in FIG. 39, the mobile station 5 selects the downlink data of the best quality from among a plurality of items of downlink data received from the base station controller 1 via the plurality of base stations 3a, 3b. That is, when the DHO state is in effect, the base station controller 1 duplicates data that has entered from a core network, which is the host network, and allocates the data to the base stations 3a, 3b. The data allocated is subjected to error correction encoding processing such as convolutional encoding, after which it is transmitted from each of the base stations 3a, 3b to the mobile station 5 utilizing a wireless section. The routes connecting the base station controller 1 to the mobile station 5 via the base stations 3a, 3b are expressed by transmission paths, and the system has two transmission paths in the illustration. Since error readily occurs in a wireless section, the mobile station 5 selects whichever of the data received from the base stations 3a, 3b has the better quality.
More specifically, as illustrated in FIG. 40, the mobile station 5 receives data from the base stations 3a, 3b via multipaths 6a1, 6a2, . . . ; 6b1, 6b2, . . . , maximum-ratio combines the data received via the multipaths on a per-base-station basis, applies error correction processing to the respective items of data that are the result of maximum ratio combining, compares the qualities with one another and selects the data having the best quality.
The foregoing relates to the case of DHO, in which the same data is transmitted from separate base stations 3a, 3b. However, handover control (sector handover: SHO) similar to DHO is carried out also in a second scheme in which, as shown in FIG. 41, the area surrounding one base station 3 is divided into sectors and directional beams are emitted from antennas AT1 to AT3 in sectors SC1 to SC3. That is, if mobile station 5 is present in sector SC1, data is transmitted only from antenna AT1. However, if the mobile station 5 moves and reaches the area at the boundary of the neighboring sector SC2, then the base station 3 transmits identical data from both antennas AT1 and AT2. In a manner similar to that of DHO, the mobile station 5 maximum-ratio combines data received via multipaths on a per-sector-antenna basis, applies error correction processing to the respective items of data that are the result of maximum ratio combining, compares the qualities with one another and selects the data having the best quality.
Such DHO and SHO control is advantageous in that reception quality can be improved. However, the following problems arise:
The first problem is that the amount of data involved in DHO increases and so does the frequency band. The fact that the same data is transmitted from the mobile station 5 to the base stations 3a and 3b or the fact that the same data is transmitted from the base stations 3a and 3b to the mobile station 5 means that the band required for transmission needs to be doubled. The principle of DHO allows a plurality of different base stations to serve as relay nodes for transmitting data. Consequently, there are cases where three or more base stations are adopted as relay nodes. The band required for transmission in such case increases by a factor of three or four, etc., namely by the number of relay nodes.
The second problem is that the amount of data involved in SHO increases and so does the frequency band. In the case of SHO, the band required for transmission needs to be doubled because the base station 3 transmits the same data to a plurality of sectors. The principle of SHO allows a plurality of different sectors to serve as transmission paths for data transmission. Consequently, there are cases where three or more sectors are adopted as transmission paths. The band required for transmission in such case increases by a factor of three or four, etc., namely by the number of transmission paths.
The third problem is an increase in transmission power, and this causes noise in other mobile stations. If the band of the data to be transmitted is large, transmission power in the wireless section is raised in a CDMA scheme in order to maintain quality. If communication is being performed via a plurality of transmission paths at the time of handover, the same band is necessary on all transmission paths. As a result, the power value used by a single mobile station rises and this causes noise in other mobile stations.
A fourth problem is excessive quality owing to superfluous power. If the quality of data from only one of the base stations 3a, 3b in the DHO state is good (i.e., if the error in the wireless section can be corrected) at the mobile station 5, then the transmission power value in the wireless section can be made the minimum necessary value. In actuality, however, it is difficult to exercise control so as to assure the quality of only one of these items data by power control. With transmission power control according to 3GPP, a transmission power adjustment (interleave power control) is carried out depending upon whether the target SIR is satisfied at each of the base and mobile stations, and control (outer-loop power control) for adjusting the target SIR depending upon the reception quality is performed. However, control is not performed so as to assure the quality of only one of the items of data. Consequently, there are cases where the qualities of both items of data that the mobile station 5 receives from the base stations 3a, 3b are good. In the selective combining method, this represents excessive quality and consumes extra power.
The fifth problem is that radio resources are exhausted. Transmitting a large quantity of data means placing a corresponding limit on spreading code that can be used simultaneously. In the case of orthogonal code used in DS-CDMA, if a short code (Walsh code) used as a channelization code is such that the spreading length (SF) is small, i.e., if a large quantity of data is about to be transmitted, other orthogonal code capable of being used simultaneously is diminished.
There is prior art (JP2000-197095A) in which a base station after handover is selected accurately by a base station controller. In this prior art, each base station detects the reliability of the receive signal based upon the TPC signal and the base station controller selects one base station based upon the reliability of each base station, applies error correction decoding processing to the receive signal that enters from this base station and decides the data.
However, this example of the prior art does not reduce the increase in amount of data and increase in necessary band in the DHO state or SHO state. Further, with this example of the prior art, transmission power is not reduced, there is no improvement in terms of excessive quality and exhausting of radio resources cannot be prevented.